A construction machine such as a hydraulic excavator is generally equipped with a hydraulic pump, a hydraulic actuator driven by a hydraulic fluid delivered from the hydraulic pump, and a flow control valve controlling the supply and discharge of the hydraulic fluid with respect to the hydraulic actuator. For example, in the case of a hydraulic excavator, the hydraulic actuators include a boom cylinder driving a boom of a front work device, an arm cylinder driving an arm, a bucket cylinder driving a bucket, a swing hydraulic motor for swinging a swing structure, a track hydraulic motor for traveling a track structure, etc., and a flow control valve is provided for each actuator. Further, each flow control valve has a meter-in restrictor and a meter-out restrictor. By the meter-in restrictor, the flow rate of the hydraulic fluid supplied from the hydraulic pump to the corresponding hydraulic actuator is controlled, and, by the meter-out restrictor, the flow rate of the hydraulic fluid discharged from the hydraulic actuator to a tank is controlled.
In a construction machine equipped with such hydraulic actuators, when the weight of the object of support of a hydraulic actuator (e.g., an arm and a bucket (attachment) in the case of an arm cylinder) acts as a load in the same direction as the operating direction of the hydraulic actuator (hereinafter also referred to as a “negative load”), the operating speed of the hydraulic actuator increases, and, as a result, there is a shortage of the flow rate of the meter-in side hydraulic fluid, resulting, in some cases, in generation of a breathing phenomenon (cavitation). As a result, there is a fear of the operability of the construction machine deteriorating.
To cope with this problem, there exists a hydraulic circuit in which there is provided a pilot type variable opening valve in a meter-out line branching off from a rod side line connected to the rod side of a hydraulic cylinder and communicating with a tank and in which the opening area of the variable opening valve is controlled in accordance with the rod side pressure (See, for example, Patent Document 1).